Marker lines are straight line segments or thicker bands that are
drawn perpendicular to a chart axis. Each line or band is drawn
outward from a specified value (for a line) or range of values (for a
band) along the axis, to highlight an arbitrary set of values.

An example would be a chart of economic statistics, where the
item-axis represents time and
has bands extending from it to mark where recessions have occurred in
the past.

There are no entries in the chart legend for marker-lines. Their meaning could be noted in
the subtitle or footnote text of a chart.

The value of the property should be a list of lists, where each
sublist is a property list that describes one line or band to draw
perpendicular to the axis. Here are the valid keys for each property
list, and the type of value to specify for each one:

:value: every sublist should have
a :value entry that specifies the point along the
axis at which to draw a line or band. The value should be a real
number (either an integer or not).

:value-to: if there is
a :value-to entry, then a band will be drawn
between the :value value to
the :value-to value. Otherwise a line will be drawn
at the :value value. The value should be a real
number (either an integer or not).

:color: this is the color in which to draw
the line or band. The value should be an RGB object
(see make-rgb) or a symbol
whose value is an RGB. When unspecified, the color will default to
red for lines and
light-gray for bands.

:line-width: this is the thickness (measured
in pixels) with which to draw the line, when
no :value-to is specified for a band. The value
should be a positive integer. When unspecified, it defaults to 1.

:line-dashing: this is the dashing style
with which to draw the line, when no :value-to is
specified for a band. The value should be one of the keyword symbols
that are valid values for the line-dashing property of a stream, such as
:solid or :dash. When
unspecified, it defaults to :solid.

Here is a sample value of the marker-lines property of an axis that would draw
two lines and two bands: